CN111600282A - Multi-terminal flexible direct-current power distribution system protection method based on weak boundary condition - Google Patents
Multi-terminal flexible direct-current power distribution system protection method based on weak boundary condition Download PDFInfo
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- CN111600282A CN111600282A CN202010463018.0A CN202010463018A CN111600282A CN 111600282 A CN111600282 A CN 111600282A CN 202010463018 A CN202010463018 A CN 202010463018A CN 111600282 A CN111600282 A CN 111600282A
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
- H02H7/26—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured
- H02H7/268—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured for dc systems
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]
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Abstract
The invention discloses a multi-terminal flexible direct current power distribution system protection method based on a weak boundary condition, which comprises the following steps: 1) in a direct current distribution system, sensors are arranged at the protective installation positions at two sides of a direct current line, and a current magnitude signal of the current line at the current level is obtained through the sensors; 2) constructing a fault area identification criterion by using the change characteristics of direct current under a fault; 3) performing wavelet change on the current magnitude signal obtained in the step 1) to obtain a current wavelet mode maximum value; 4) constructing identification criteria of fault types of a positive line, a negative line and a positive-negative line in a fault area; 5) according to the method, the problem that a multi-terminal flexible direct-current power distribution system lacks a direct-current filter as a direct-current line boundary and cannot carry out fault protection depending on the direct-current line boundary characteristics can be effectively solved.
Description
Technical Field
The invention belongs to the technical field of relay protection of power systems, and relates to a multi-terminal flexible direct-current power distribution system protection method based on a weak boundary condition.
Background
With the increasing proportion of distributed generation in electric energy in recent years, the number of direct current loads is increasing. When the traditional alternating current power distribution system is used for realizing the consumption of distributed energy and the power supply of a direct current load, a large amount of power electronic current conversion equipment is needed, and the investment cost is greatly improved. On the contrary, if the consumption of the distributed energy and the power supply of the direct current load are carried out through the direct current distribution system, a large amount of current conversion equipment can be saved, and the investment cost is reduced. In addition to the economic advantages, the dc distribution system has the following advantages compared to the conventional ac distribution system: improving power supply reliability, improving power quality and the like. Meanwhile, in recent years, the flexible commutation technology is gradually mature, and the rapid development of the direct-current power distribution network is promoted. Therefore, the flexible dc power distribution system becomes one of the mainstream trends of the future energy internet development. The multi-end flexible direct-current power distribution network has obvious advantages in the aspects of power supply reliability and the like, and is a research and application hotspot of the direct-current power distribution network.
The protection system of the multi-end flexible direct-current power distribution network is an important guarantee for rapidly and reliably removing faults, reducing the fault influence range and ensuring the safe and stable operation of the power distribution network. In a direct-current distribution system, the damping value of a direct-current line is small, once a fault occurs in a direct-current field, all direct-current lines can rapidly overflow, and therefore the direct-current distribution network is greatly damaged, and therefore a protection system is an important component in research work of a flexible direct-current distribution network.
The multi-terminal flexible direct current topology has the characteristic of reducing transmission loss, and has smaller electric shock and lower electromagnetic field beneficial to electromagnetic compatibility. Fragile VSC converter valves are more vulnerable to dc faults, especially when the converter is of MMC topology, and in addition one fault can cause overvoltage and overcurrent in the whole network, making its protection more complex. The protection of a direct current system is different from the protection of an alternating current system, and because a direct current power grid is a low-damping system, the fault electrical quantity rises very fast, higher requirements on the quick action of protection are provided, and the application of the traditional impedance protection is also limited. Most of protection methods proposed by the direct current system are applicable to a point-to-point direct current system, but cannot be extended to a multi-terminal flexible direct current system, so that design and research on the multi-terminal flexible direct current protection system are necessary. Although some converter topologies like full-bridge MMC have a dc fault current blocking function; several vector control methods can also be applied to VSC converters, which also contribute to the fault blocking process, but any need for a suitable protection algorithm to detect the fault occurrence within a few milliseconds of the fault occurrence and to distinguish and isolate the fault part from the healthy network part.
The existing multi-terminal flexible direct current protection research algorithms at home and abroad can be generally divided into two types, one is a protection criterion constructed based on voltage, current derivative, traveling wave and the like, and the other is a multi-terminal flexible direct current system protection algorithm designed based on digital signal processing methods such as wavelet/fast Fourier technology, mathematical morphology, Hilbert-Huang transform and the like. In summary, the current research on the multi-terminal flexible dc protection has the problems of poor selectivity, low sensitivity, and incapability of ensuring reliability in the dc system under the weak boundary condition of no dc filter with very small dc reactance.
At present, aiming at multi-terminal flexible direct current protection research under a weak boundary condition, electrical quantity change under a fault needs to be analyzed to construct a protection partition criterion so as to avoid partition difficulty caused by flexible power reversal and the weak boundary condition; and pole selection criteria suitable for the high-resistance grounding fault condition are given according to the electrical quantity characteristics detected under different faults. And finally, verifying the applicability of the protection strategy in the multi-terminal direct current network under the weak boundary condition through simulation.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides a method for protecting a multi-terminal flexible direct-current power distribution system based on a weak boundary condition, which can effectively solve the problem that the multi-terminal flexible direct-current power distribution system lacks a direct-current filter as a direct-current line boundary and cannot perform fault protection by depending on the direct-current line boundary characteristics.
In order to achieve the above object, the method for protecting a multi-terminal flexible dc power distribution system based on weak boundary conditions according to the present invention comprises the following steps:
1) in a direct current distribution system, sensors are arranged at the protective installation positions at two sides of a direct current line, and a current magnitude signal of the current line at the current level is obtained through the sensors;
2) constructing a fault area identification criterion by using the change characteristics of direct current under a fault;
3) performing wavelet change on the current magnitude signal obtained in the step 1) to obtain a current wavelet mode maximum value;
4) analyzing the maximum values of the current wavelet modes under different fault types, and constructing identification criteria of fault types of a positive line, a negative line and a line between the positive line and the negative line in a fault area;
5) and performing a criterion process suitable for protecting the multi-terminal flexible direct-current line under the weak boundary condition according to the identification criterion of the fault area and the identification criteria of the fault types of the positive line, the negative line and the line between the positive and negative poles in the fault area.
Calculating a value γ, wherein the value γ is:
wherein i + rec is the positive current of the rectifier station, i + inv is the positive current of the inverter station, and t is time;
when an intra-area fault occurs, the variation trend of i + rec is the same as that of i + inv, and gamma is greater than 0; when an out-of-range fault occurs, i + rec and i + inv change in opposite directions, where γ < 0.
The fault identification criterion of the positive line in the construction area is as follows:
the fault identification criterion of the negative line is as follows:
the fault identification criterion of the line between the positive pole and the negative pole is as follows:
wherein k is1And k2Is a coefficient determined by the refractive index and the reliability margin, WijRepresents the current wavelet transform modulus maximum, W, of the i-terminal protection installation on the ij linejiAnd the current wavelet transformation modulus maximum value at the j-end protection installation position on the ij line is shown.
The invention has the following beneficial effects:
when the multi-terminal flexible direct current power distribution system protection method based on the weak boundary condition is operated specifically, the change characteristics of direct current under the fault are utilized to construct fault area identification criteria; the method comprises the steps of analyzing maximum values of current wavelet modes under different fault types, constructing identification criteria of fault types of a positive line, a negative line and a positive-negative line in a fault area, and performing a criterion flow suitable for protecting the multi-terminal flexible direct-current line under weak boundary conditions according to the identification criteria of the fault area and the identification criteria of the fault types, so that the problem that the multi-terminal flexible direct-current power distribution system is lack of a direct-current filter as a direct-current line boundary and cannot be subjected to fault protection by means of direct-current line boundary characteristics is effectively solved.
Drawings
FIG. 1 is a topology diagram of a four-terminal flexible DC power distribution system;
FIG. 2 is a schematic diagram of a fault of an N-terminal flexible DC distribution network;
FIG. 3 is a flow chart of a directional pilot protection criterion based on current polarity;
FIG. 4 is a schematic diagram of a network fault component of a four-terminal flexible DC power distribution system;
FIG. 5a is a gamma waveform illustrating a fault in line 34;
FIG. 5b is a gamma waveform for an out-of-range fault on line 34;
FIG. 6a is a diagram of the original waveform of the positive short-circuit ground of the line 34;
FIG. 6b is the original waveform of the short-circuit grounding of the positive electrode of the line 43;
FIG. 6c is a schematic diagram of the wavelet transform modulus maxima when the positive electrode of line 34 is shorted to ground;
FIG. 6d is a schematic diagram of the maximum of the wavelet transform modulus when the positive electrode of the line 43 is short-circuited and grounded;
FIG. 7a is a diagram of the original waveform of the short-circuit ground of the negative electrode of the line 34;
FIG. 7b is the original waveform of the short-circuit grounding of the negative electrode of the line 43;
FIG. 7c is a schematic diagram of the wavelet transform modulus maximum when the negative electrode of line 34 is short-circuited and grounded;
FIG. 7d is a schematic diagram of the maximum of the wavelet transform modulus when the negative electrode of the line 43 is grounded;
FIG. 8a is a diagram of an original waveform of the short circuit between the electrodes of the line 34;
FIG. 8b is a diagram of an original waveform when the line 43 is short-circuited between poles;
FIG. 8c is a schematic diagram of the wavelet transform modulus maxima during an inter-electrode short circuit of the lines 34;
fig. 8d is a schematic diagram of the wavelet transform modulus maximum in the case of an inter-electrode short circuit of the line 43.
Detailed Description
The invention is described in further detail below with reference to the accompanying drawings:
the invention mainly utilizes the characteristics of wavelet transformation modulus maximum of direct current fault current traveling waves to realize the judgment of protection partitions and fault types, and particularly relates to a multi-terminal flexible direct current power distribution system protection method based on weak boundary conditions, which comprises the following steps:
1) in a direct current distribution system, sensors are arranged at the protective installation positions at two sides of a direct current line, and a current magnitude signal of the current line at the current level is obtained through the sensors;
2) constructing a fault area identification criterion by using the change characteristics of direct current under the fault as shown in figure 2;
3) performing wavelet change on the current magnitude signal obtained in the step 1) to obtain a current wavelet mode maximum value;
4) analyzing the maximum values of the current wavelet modes under different fault types, and constructing identification criteria of fault types of a positive line, a negative line and a line between the positive line and the negative line in a fault area;
5) and performing a criterion process suitable for protecting the multi-terminal flexible direct-current line under the weak boundary condition according to the identification criterion of the fault area and the identification criteria of the fault types of the positive line, the negative line and the line between the positive and negative poles in the fault area.
During the specific operation of the step 2), calculating a numerical value gamma, wherein the numerical value gamma is as follows:
wherein i + rec is the positive current of the rectifier station, i + inv is the positive current of the inverter station, and t is time;
when an intra-area fault occurs, the variation trend of i + rec is the same as that of i + inv, and gamma is greater than 0; when an out-of-range fault occurs, i + rec and i + inv change in opposite directions, where γ < 0.
Referring to fig. 3, for a positive line fault, it is equivalent to superimposing a negative dc voltage source at the fault point, the traveling wave current propagates to both ends of the line at a corresponding wave speed, and the traveling wave head is smaller than the positive pole when the traveling wave is refracted to the protection of the negative pole due to the different wave impedances in the converter station, so the fault identification criterion of the positive line in the constructable area is:
similarly, the fault identification criterion of the negative line is as follows:
the fault identification criterion of the line between the positive pole and the negative pole is as follows:
wherein k is1And k2Is a coefficient determined by the refractive index and the reliability margin, WijRepresents the current wavelet transform modulus maximum, W, of the i-terminal protection installation on the ij linejiAnd the current wavelet transformation modulus maximum value at the j-end protection installation position on the ij line is shown.
The verification of the invention takes a four-terminal flexible direct-current annular power distribution network as an example, as shown in fig. 1, wherein a line 34 is taken as an object, and an intra-area fault of a positive short-circuit ground, a negative short-circuit ground and an inter-positive short-circuit fault type is set on the line, at this time, the line 3i, 4j and ij and an intra-converter station fault are considered as an extra-area fault for simulation verification. Referring to fig. 5a to 8d, fig. 5a to 5b show the gamma value change obtained by setting the same positive ground fault on the line 34 and the line 31, respectively. And when the positive pole of the line 34 has a fault, the pole selection criterion judges that the fault type is the verification of the positive pole ground fault. And when the negative pole of the line 34 has a fault, the pole selection criterion is used for judging that the fault type is the verification of the negative pole grounding fault. When the line 34 has a fault between the positive pole and the negative pole, the judgment of the fault type by the pole selection criterion is verified to be the fault between the positive pole and the negative pole, and the table 1 shows the wavelet modulus maximum value and the fault judgment condition under the fault conditions of different transition resistances.
TABLE 1
Wherein, the fault 1 is a positive earth fault of a 31-line circuit; the fault 2 is a positive fault of 34 lines, and the fault 3 is a negative ground fault of 34 lines; fault 4 is a 34-line inter-pole short circuit fault.
The above results show that the present invention can effectively determine the fault region and the fault pole, and has sufficient sensitivity under the fault of high transition resistance.
The invention can ensure that the fault is correctly partitioned without depending on the boundary condition of the direct current line, and the protection method is not influenced by high transition resistance, can well judge the fault conditions of different types, has good reliability and provides reference for the development of the protection of the multi-end flexible direct current distribution line.
Claims (3)
1. A multi-terminal flexible direct current power distribution system protection method based on weak boundary conditions is characterized by comprising the following steps:
1) in a direct current distribution system, sensors are arranged at the protective installation positions at two sides of a direct current line, and a current magnitude signal of the current line at the current level is obtained through the sensors;
2) constructing a fault area identification criterion by using the change characteristics of direct current under a fault;
3) performing wavelet change on the current magnitude signal obtained in the step 1) to obtain a current wavelet mode maximum value;
4) analyzing the maximum values of the current wavelet modes under different fault types, and constructing identification criteria of fault types of a positive line, a negative line and a line between the positive line and the negative line in a fault area;
5) and performing a criterion process suitable for protecting the multi-terminal flexible direct-current line under the weak boundary condition according to the identification criterion of the fault area and the identification criteria of the fault types of the positive line, the negative line and the line between the positive and negative poles in the fault area.
2. The method for protecting the multi-terminal flexible direct current power distribution system based on the weak boundary condition of claim 1, wherein a value γ is calculated, wherein the value γ is:
wherein i + rec is the positive current of the rectifier station, i + inv is the positive current of the inverter station, and t is time;
when an intra-area fault occurs, the variation trend of i + rec is the same as that of i + inv, and gamma is greater than 0; when an out-of-range fault occurs, i + rec and i + inv change in opposite directions, where γ < 0.
3. The multi-terminal flexible direct current power distribution system protection method based on the weak boundary condition of claim 1, wherein the fault identification criteria of the positive line in the construction area are as follows:
the fault identification criterion of the negative line is as follows:
the fault identification criterion of the line between the positive pole and the negative pole is as follows:
wherein k is1And k2Is a coefficient determined by the refractive index and the reliability margin, WijRepresents the current wavelet transform modulus maximum, W, of the i-terminal protection installation on the ij linejiAnd the current wavelet transformation modulus maximum value at the j-end protection installation position on the ij line is shown.
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